Mechanism of unloading-induced permeability increment of protected coal seam under mining

Protective coal seam mining has great significance in coal production as a typical form of safe mining of coal and gas simultaneously. The displacement field expression within the rock mass can be solved by means of the semi-infinite integral model, and the actual deformation of coal seam can be better explained by the theoretical model through contrast to subsidence curves of protected coal seam of similar simulation tests. A two-zone crack distribution model as well as the simplified mechanical model is established. It is found that there are three processes of gas permeability from the orthogonal design of the complete stress-strain penetration tests, which has a dramatic change in the volumetric strain of 0.015. The law of symmetrical distribution of volumetric strain along settlement range can be obtained by comparing the theoretical volumetric strain distribution curve, which has a sharp increasing penetration rate in the range, which is larger than 0.015. The results will be useful for the calculation of gas permeability induced by the pressure relief of protected coal seam.